We can speculate that possibly these spoilers are from a different version of the Guardians of the Galaxy 2 script or the leaker changed a few things as to not spoil the entire movie.

I suppose another possibility is that the leaker is just good at guessing.

Note: rumor in bold with my comments after:

Months after saving Xandar, the Guardians of the Galaxy - Star-Lord (Chris Pratt), Gamora (Zoe Saldana), Drax (Dave Bautista), Rocket (Bradley Cooper) and Baby Groot (Vin Diesel) - have become heroes for hire, and are employed to destroy an extradimensional monster, but nearly botch their mission due to their in-fighting.

Sounds about right which is the start of the movie.

The Kree empire has placed a bounty on their heads for the death of Ronan, and Star-Lord's surrogate father, Yondu Udonta (Michael Rooker), is betrayed by one his enforcers, Taserface (Chris Sullivan), who seizes control of the Ravagers and seeks to capture the Guardians.

Yondu does get betrayed by Taserface, as Taserface mutinies and does gain control of the Ravagers; however no Kree or mention of Ronan.

The Guardians are hired by Ayesha (Elizabeth Debicki), a member of an ancient race known as "The Eternals", to find a rogue member of their group, Ego (Kurt Russell). The Guardians are then attacked by the Ravagers and become separated, with Rocket and Baby Groot being captured by the Ravagers while Star-Lord, Gamora and Drax make it to the planet where Ego might be.

Rocket and Groot do get captured by the Ravagers along with Yondu, while the rest of the Guardians go with Ego. Also the Guardians were hired by Ayesha, but to kill the extra-dimensional creature, not go after Ego.

They meet Mantis (Pom Klementieff), a clairvoyant and the planet's sole inhabitant. Star-Lord finds out Ego is an ancient force that inhabits the planet's core, filling it with life, and was also the "angel" that seduced his mother Meredith Quill during one of his many voyages through Space. Ego is therefore his father, and Mantis is his half-sister.

Mantis is the sole inhabitant of Ego; Ego does inhabit the core.

Meanwhile, Rocket and Baby Groot come across Yondu and Nebula (Karen Gillian), who has also been captured by the Ravagers. They work together in order to escape.

Sounds about right.

Ego convinces Quill that the Eternals are planning to conquer the galaxy and need to be stopped. Quill and Gamora face their blossoming relationship, and the team regroups to confront the Eternals.

Not exactly right as there are no Eternals in the movie. Ego calls himself a Celestial, and it's Ego that is trying to conquer the galaxy/universe.

It turns out Ego has manipulated them and is the real menace, with the Eternals being the only ones standing on his way. The Guardians then rally to fight both Ego and the arriving Ravagers. The Kree empire and the Nova Corps become involved as well.

Ego is revealed to be the main bad guy, but no Nova Corps or Kree are in the movie (Gunn did shoot a scene/s with Glenn Close which may make it into the Blu-Ray).

Yondu sacrifices himself to destroy Ego, and the Guardians learn how to overcome their differences to save the Eternals. Mantis joins the team, and Nebula goes off on her own, seeking revenge against her adoptive father, Thanos (Josh Brolin). Taserface is killed, and the Ravagers are taken over by Yondu's right-hand man, Kraglin (Sean Gunn) and go back to their old ways.

Yondu does die; Mantis does join the team; Nebula does go off to find and kill Thanos; Taserface dies, and we can guess Kraglin may eventually take over the Ravagers (or form a new team of them).